1. Field of the Invention
The present invention relates to a device for preventing the liquefaction-leakage phenomenon of gas in a pressure regulator.
2. Description of the related art
For doping gases in the manufacture of semiconductors, for instance, it is known to use diborane gas (B.sub.2 H.sub.6) or a gas mixture of diborane gas and argon, nitrogen and/or silane gas. As shown on FIG. 1, such gas is usually stored under high pressure in a cylinder 1, for example pure diborane gas at about 30 bar and a mixed gas of diborane gas and argon at a higher pressure of 100 bar. At a feed installation, the pressure of said doping gas is reduced to several bars by the use of a pressure regulator 2.
When the pressure of a high pressure gas, such as a doping gas, is reduced by a using a pressure regulator, said gas is cooled and might be at least partially liquefied because of its free expansion through the pressure regulator. This can generate some leaks out of the valve seal of the pressure regulator. This situation is known as a liquefaction-leakage phenomenon of gas which makes the working of a pressure regulator unsafe, As diborane gas or the like is a very poisonous gas, this kind of leakage can generate important environmental problems.
It has already been proposed in Japanese Patent Application Kokai No. 238,800/1989, a solution to this leakage problem as represented on FIG. 2: a gas pipe 3 connecting a gas cylinder 1 containing said doping gas and a pressure regulator 2 is heated by a heating device 4 provided around said gas pipe 3. This heating device 4 is controlled by an automatic controller 5 which controls the heat transferred to the gas pipe 3 so that the feed gas is heated in the heating device 4. This feed gas is heated at a temperature which is such that the temperature of the gas at outlet 7 after expansion through the pressure regulator 2 is at least equal to or greater than the temperature of the feed gas in the pipe 3 before heating by said device 4. As a result, the temperature of the feed gas from the gas cylinder 1 is made almost equal to that of the gas at the gas outlet 7 of the pressure regulator 2, thereby preventing the liquefaction-leakage of the feed gas.
In such a conventional device, however, it is difficult to carry out the temperature control of the gas pipe 3, i.e. the temperature of the feed gas by the heating device 4. It is furthermore difficult to detect whether the feed gas leaks from the gas pipe 3 through pin holes formed by corrosion.
The present invention is intended to eliminate the aforementioned drawbacks.